The optical circulator is an optical device having a light separating function to output an input light from a certain port only to another particular port (e.g., output an input light from a first port P1 to a second port P2, and output an input light from a second port P2 to a third port P3). Meanwhile, the optical switch is an optical device having an optical-path switching function to output an input light from a first input port I1 to either a first output port O1 or a second output port O2, and output an input light from a second input port I2 to either a second output port O2 or a first output port O1.
Usually, the optical circulator uses a 45-degree Faraday rotator for applying a fixed magnetic field by a permanent magnet. By rotating the polarization plane 45 degrees in a predetermined direction, the non-reciprocality of a light ray is realized. Although the optical switch realizes optical-path switching by the use of a variable Faraday rotator for changing the direction of an applied magnetic field due to an electromagnet, basic parts can be structured nearly in a similar manner.
Conventionally, there have been developed various structures of optical circulators and optical switches. There is, as one example of those, a structure in which four birefringent elements are arranged in one row with spacing, and sets of a Faraday rotator and a ½-wavelength plate are respectively inserted between the birefringent elements, to provide ports at both ends (e.g. see JP-A-2000-89164). Due to this, an optical circulator can be structured which couples input light from a first port positioned at one end to a second port at the opposite end, input light from a second port to a third port at its opposite end, input light from a third port to a fourth port at its opposite end, and input light from the fourth port to the first port at its opposite end, respectively.
In the above conventional structure, the ½-wavelength plate in a part (two plates positioned nearby the both ends) uses a combination, for each optical path, of four ½-wavelength plates different in the optical-axis direction. Also, used in the arrangement center are ½-wavelength plates in combination arranged side by side with two-piece spacing. Thus, the structure is complicated and the parts are many in the number.